Polytetrafluoroethylene (PTFE) is produced in mainly two types of processes: suspension polymerization and aqueous dispersion (emulsion) polymerization. Aqueous dispersion (emulsion) polymerization provides a latex, and polymer fine particles therein are aggregated and dried to obtain a powder (fine powder), which is then processed by paste extrusion after being compounded mainly with a liquid lubricant. By paste extrusion, the compound is extruded into a form of a rod or a tube from an extruder. The extruded articles are used as sealing materials without being subjected to further processing or by being rolled into a sheet without sintering (calendering), or as filter materials by being stretched into porous films, or are further sintered to be used as molded articles such as tubes, and wire-coatings.
When a high shear force is applied to PTFE aqueous dispersions, primary particles are aggregated and secondary particles having an average particle size of 100 to 1000 μm are formed (coagulation). It is known that in order to promote coagulation, an additive such as acids, alkalis, organic solvents is added. A secondary particle powder obtained by coagulation of PTFE aqueous dispersions is dried to give a PTFE fine powder.
PTFE fine powders are characterized by their fibrillating properties. By paste extrusion or calendering as described above, molded articles such as sealing materials, filters, tubes, pipes, and wire-coatings can be produced.
Although the above properties of PTFE fine powders may be advantageous, some problems may also be caused such that secondary particle powders tend to be aggregated due to fibrillation and that the formed aggregates may not restore themselves to their original secondary particle powders. Because of these problems, PTFE fine powders tend to be aggregated or aggregated to be consolidated during production, transport or molding processes thereof, impairing handling characteristics thereof.
The aggregates may lead to reduced molding properties and deteriorated appearance of molded articles. Thus, it is usually necessary to remove the aggregates with a sieve during production or before molding. They may lead to decreased working efficiency or productivity.
As one of methods for manufacturing a PTFE fine powder which can solve the above problems and is excellent in powder flowability, Patent Literature 1 discloses a method for manufacturing a fine powder having a high apparent density and excellent flowability by adding a fluorine-containing anionic surfactant to a PTFE aqueous dispersion. In Patent Literature 1, coagulation of the PTFE aqueous dispersion is carried out by agitation using an anchor-type impeller.
Patent Literature 2 discloses granulation of a modified PTFE powder obtained by suspension polymerization, which is carried out in a granulation tank equipped with a cone-shaped impeller. Patent Literature 3 discloses suspension polymerization in a 150-L stainless steel polymerization vessel equipped with a cone-shaped impeller. Neither literature discloses, however, coagulation in a PTFE aqueous dispersion obtained by emulsion polymerization, which is carried out by agitation using a cone-shaped impeller.